DESCRIPTION: The membrane of the human red cell has been embraced by cell biologists as a simple model with direct relevance to the complex membranes of non-erythroid cells. While the structural biology of the ankyrin-band 3 end of the spectrin cytoskeleton have been thoroughly studied, the opposite end of the spectrin cytoskeleton has remained puzzling and has proven to be a complex of multiple polypeptides. The applicant proposes to elucidate the molecular determinants of actin filament length in red cells and certain muscle cells. Addition and removal of actin from the barbed ends of filaments has been studied extensively, but the determinants of pointed end growth are not well understood. Previous work by this applicant led to the identification and characterization of tropomodulin in red cells. In the presence of tropomyosin, tropomodulin was shown to cap the pointed ends of actin filaments. The investigator proposes a series of cellular and molecular biological studies to further explain this process. She plans to map the filament and pointed end binding domains on tropomodulin and explore the contributions of tropomyosin to this association. Using cultured cardiac and skeletal myogenic cells, she will examine the effects of varying tropomodulin concentrations and will attempt to block specific domains with injected antibodies. She will search for and characterize new non-erythroid tropomodulin isoforms. The roles of adducing, eCP, and tropomodulin will be evaluated by mechanically stressing red cell membranes.